New size improvements in the GWT compiler produce JavaScript that is 10-20% smaller when simply recompiled with GWT 1.4 RC.

An enhanced startup sequence reduces the size of your module's startup script by 80%! Even better, the new startup sequence removes an entire HTTP round-trip, reducing latency during those crucial first few hundred milliseconds at startup. According to our measurements, modules load about 33% faster as a result.

The above optimizations combined with ImageBundle, described below, make it possible for GWT-based applications to load surprisingly quickly. To see for yourself, check out startup time of the Mail sample included in the GWT 1.4 RC download.

GWT RPC is no longer tied to exclusively to servlets. New modularized RPC server code makes it easy to connect GWT RPC to your choice of Java back-ends.

SuggestBox makes it easy to add auto-complete functionality.

Splitters! HorizontalSplitPanel and VerticalSplitPanel allow users to resize portions of the user interface.

A new benchmarking subsystem integrates with JUnit to let you record and compare the speed of code snippets across multiple browsers and multiple parameter ranges. Benchmarking is a powerful way to identify bottlenecks and compare performance of alternative implementations.

The oft-requested java.io.Serializable is now included in the JRE emulation library and is synonymous with IsSerializable for the purpose of GWT RPC.

Mouse wheel events are now available on a variety of widgets.

Johnson notes that he feels ImageBundle is the single biggest feature of the RC release. The feature allows GWT to combine dozens of images into one "strip". This collapses them into one cachable HTTP request instead of what would have been numerous requests. Another benefit of using image bundles is the elimination of "bouncy" relayout as images are loaded and widths/heights are adjusted.